Conventional video compression removes both temporal redundancy (similarities between adjacent frames) and spatial redundancy (similarities within a single frame) from video in order to reduce the amount of data that is stored and/or transmitted. In the conventional digital transmission paradigm, transmission is assumed to be lossless as long as channel coding provides sufficient protection (e.g., error correction). However, video degradation in noisy environments is not graceful; at low Signal-to-Noise Ratio (SNR), the video cannot be reproduced resulting in choppy or stalled video reproduction at the receiver.
In a unicast arrangement the transmitter adjusts video quality to accommodate the receiver's channel condition. In a multicast environment the transmitter either sends individual transmissions to all receivers (to accommodate their individual channel conditions), or the transmitter transmits video according to the worst channel condition of all receivers. The first method increases bandwidth usage while the second method results in reduced video quality.
An analog video system named SoftCast has been proposed by S. Jakubczak and D. Katabi. (“A cross-layer design for scalable mobile video,” In Proceedings of the 17th annual international conference on Mobile computing and networking, MobiCom '11, pages 289-300.) In Softcast 3D-DCT (Discrete Cosine Transform) is performed on a group of pictures, and the transform coefficients are transmitted as the I and Q components of a complex symbol after power scaling. Most of the temporal and spatial redundancy is retained. Such a scheme is capable of achieving graceful degradation in a wide range of channel conditions, making it suitable for multicast.